[host] app: calculate and report the required IVSHMEM size

One of the most common issues reported in the support channels is the
IVSHMEM size being too small. This change adds a calculation to
determine an optimal size and uses the new `os_showMessage` platform
method to display a message box to the user with the error.

host/platform/Windows/src/platform.c

@@ -537,3 +537,8 @@ bool os_blockScreensaver()
   }
   return lastResult;
 }
+
+void os_showMessage(const char * caption, const char * msg)
+{
+  MessageBoxA(NULL, msg, caption, MB_OK | MB_ICONINFORMATION);
+}